Polarization beam splitter

ABSTRACT

Consistent with the present disclosure, a polarization multiplexed optical signal having optical signals with both TE and TM polarizations is supplied to an input of a polarization beam splitter (PBS). The PBS includes a first output that supplies TE polarized optical signals and a second output supplies TM polarized optical signals. A first polarizer is coupled to the second output of the PBS to pass the TM polarized optical signals, while rejecting light having other polarizations, such as the TE polarization. A rotator then rotates the light output from the first polarizer, so that such light has a TE polarization. A second polarizer is coupled to the rotator to filter light having a polarization other than the TE polarization. In addition, a third polarizer is coupled to the first output of the PBS in order to filter or block any TM light, for example, that may be output from the PBS with the TE polarized signal. Accordingly, the optical signals in the polarization multiplexed optical signal are effectively filtered, and some are rotated so that each has the same, e.g., TE polarization. Moreover, each optical signal is substantially free from light having extraneous polarizations and may be supplied to an optical hybrid circuit. Data carried by such optical signals may thus be reliably recovered.

The present application claims the benefit of U.S. ProvisionalApplication No. 61/219,725, filed Jun. 23, 2009, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

Wavelength division multiplexed (WDM) optical communication systems areknown in which multiple optical signals, each having a differentwavelength, are combined onto a single optical fiber. Such systemstypically include transmitters having a laser associated with eachwavelength, a modulator configured to modulate the output of the laser,and an optical combiner to combine each of the modulated outputs.Receivers are also provided to demultiplex the received WDM signal intoindividual optical signals, convert the optical signals into electricalsignals, and output the data carried by those electrical signals.

Conventionally, WDM systems have been constructed from discretecomponents. For example, demultiplexer and photodiodes have be packagedseparately and provided on a printed circuit board. More recently,however, many WDM components, have been integrated onto a single chip,also referred to a photonic integrated circuit (PIC).

In order to further increase the data rates associated with WDM systems,various modulation formats have been proposed for generating themodulated optical output.

One such optical signal modulation format, known as polarizationmultiplexed differential quadrature phase-shift keying (“Pol MuxDQPSK”), can provide spectral densities with higher data rates per unitof fiber bandwidth than other modulation formats, such as on-off keying(OOK).

A receiver configured to decode and output the information carried by aPol Muxed DQPSK signal is described in U.S. patent application Ser. Nos.12/052,541; 12/345,817; and 12/345,824, the entire contents of each ofwhich are incorporated herein by reference. In such systems, variouscomponents are provided on multiple substrates. There is a need,however, to provide such components of an optical receiver, whichreceives Pol Muxed DQPSK modulated optical signals or optical signalsmodulated in accordance with other modulation formats, on one substrateto improve reliability, simplify manufacturing, and reduce costs.

In addition, a polarization beam splitter may be provided that receivesoptical signals having first and second polarizations at an input port,and outputs the optical signals having one polarization, e.g., a TEpolarization, on a first output, and those optical signals having theother polarization, e.g., TM polarization, are output separately on asecond output port. In a DQPSK system, for example, the optical signalsmay be supplied to a known optical hybrid circuit, which, as generallyunderstood, outputs components, such as the quadrature (Q) and in-phase(I) of the input optical signals. As is further generally understood,the polarization states of the incoming optical signals supplied to theoptical hybrid circuit are preferably the same, in order to adequatelyrecover the data carried by the optical signals. Thus, one of the TE andTM signals output from the polarization beam splitter, e.g., the TMsignal, is supplied to a rotator so that the TM polarization is rotatedto a TE polarization state. As a result, both signals supplied to theoptical hybrid circuit nominally have the same TE polarization.

Often, however, undesired residual or extraneous TM light may beincluded with the TE light output from one port of the polarization beamsplitter, and similarly, a relatively small amount of TM light may beoutput from the rotator. Such residual TM light may result in errors indata recovered from the optical signals. Accordingly, there is a need toreduce or suppress such residual TE and TM light.

SUMMARY

Consistent with an aspect of the present disclosure, an apparatus isprovided that includes a polarization beam splitter having an inputwaveguide and first and second output waveguides. The polarization beamsplitter is configured to receive, at the input waveguide, a firstoptical signal having a first polarization and a second optical signalhaving a second polarization different than the first polarization. Afirst polarizer is also provided that is configured to pass light havingthe first polarization, the first polarizer being configured to receivethe first optical signal from the first output of the polarization beamsplitter. In addition, a rotator is provided that is configured toreceive the first optical signal from the first polarizer. The rotatoris configured to output the first optical signal so that the firstoptical signal has the second polarization. A second polarizer is alsoincluded that is configured to pass light having the secondpolarization. The second polarizer is configured to receive the firstoptical signals having the second polarization. Further, a thirdpolarizer is provided that is configured to pass the light having thesecond polarization. The third polarizer is configured to receive thesecond optical signal having the second polarization. The first opticalsignal is one of a plurality of first optical signals and the secondoptical signal is one of a plurality of second optical signals. Each ofthe plurality of first optical signals has a corresponding one of aplurality of wavelengths, and each of the plurality of second opticalsignals having a corresponding one of the plurality of wavelengths. Eachof the plurality of first optical signals is output, with the secondpolarization, from the second polarizer and each of the plurality ofsecond optical signals is output, with the second polarization, from thethird polarizer. Moreover, optical demultiplexer circuitry is providedthat has first and second inputs. The first input of the opticaldemultiplexer circuitry is configured to receive the plurality of firstoptical signals from the second polarizer and the second input of theoptical demultiplexer circuitry is configured to receive the pluralityof second optical signals from the third polarizer

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, together with the description, serve toexplain the principles of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of An optical receiver consistent with anaspect of the present disclosure;

FIG. 2 illustrates a polarization beam splitter and polarizersconsistent with an additional aspect of the present disclosure;

FIGS. 3-12 illustrate examples of optical demultiplexers consistent withthe present disclosure.

FIG. 13 illustrates an example of a receiver circuit consistent with anaspect of the present disclosure;

FIGS. 14 a-14 e illustrate additional examples of a receiver circuitconsistent with an aspect of the present disclosure; and

FIG. 15 illustrates an example of an optical multiplexer consistent withthe present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Consistent with the present disclosure, a polarization multiplexedoptical signal having optical signals with both TE and TM polarizationsis supplied to an input of a polarization beam splitter (PBS). The PBSincludes a first output that supplies TE polarized optical signals and asecond output supplies TM polarized optical signals. A first polarizeris coupled to the second output of the PBS to pass the TM polarizedoptical signals, while rejecting light having other polarizations, suchas the TE polarization. A rotator then rotates the light output from thefirst polarizer, so that such light has a TE polarization. A secondpolarizer is coupled to the rotator to filter light having apolarization other than the TE polarization. In addition, a thirdpolarizer is coupled to the first output of the PBS in order to filteror block any TM light, for example, that may be output from the PBS withthe TE polarized signal. Accordingly, the optical signals in thepolarization multiplexed optical signal are effectively filtered, andsome are rotated, so that each has the same, e.g., TE polarization.Moreover, each optical signal is substantially free from light havingextraneous polarizations and may be supplied to an optical hybridcircuit. Data carried by such optical signals may thus be reliablyrecovered.

Reference will now be made in detail to the present exemplaryembodiments, which are illustrated in the accompanying drawings.

FIG. 1 illustrates a block diagram of an optical receiver circuit 100consistent with the present disclosure. Optical receiver circuit 100includes polarization demultiplexer 112 provided on substrate 110. A PolMuxed DQPSK wavelength division multiplexed (WDM) signal may be providedto polarization demultiplexer 112. The Pol Muxed DQPSK signal mayinclude first optical signals, each of which having a first polarization(e.g., TE) and a corresponding one of wavelengths λ1 to λn, and secondoptical signals, each of which having a second polarization (e.g., TM)and a corresponding one of the wavelengths λ1 to λn. The first opticalsignals may be output from a first port of polarization demultiplexer112 and the second optical signals may be output from a second port ofpolarization demultiplexer 112 (not shown in FIG. 1, but discussed ingreater detail below). The first and second optical signals may beamplified by optical amplifiers 114 and supplied to demultiplexing anddelay circuitry 116, which optically demultiplexes the received signalsby wavelength (typically without conversion to electrical signals) anddelays the optical signals relative to one another for furtherprocessing by known optical hybrid circuits 118. Optical hybrid circuits120, in turn, provide additional optical signals to correspondingphotodiodes 120. A processor circuit 122 receives and processes theelectrical outputs from photodiodes 120 with multi-input-multiple-output(MIMO) circuitry, as described, for example, in the above-noted U.S.patent application Ser. No. 12/052,541. The electrical outputs fromprocessor circuit 122 may be supplied to clock and data recoverycircuitry and forward error correction (FEC) decoding circuitry (notshown).

Polarization demultiplexer 112 will next be described with reference toFIG. 2. Polarization demultiplexer 112 includes a polarization splitter,also referred to as a polarization beam splitter (PBS) 202, asdescribed, for example, in Soldano et al., “Mach-Zehnder InterferometerPolarization Splitter in InGaAsP/InP”, IEEE Photonics TechnologyLetters, vol. 6, no. 3, March 1994, pp. 402-405, the entire contents ofwhich are incorporated herein by reference. PBS 202 may constitute aMach-Zehnder interferometer including an input waveguide 201, and abranch waveguide 203 having one or more layers of metal or othermaterials that affect the index of light having either a TM (“transversemagnetic”) polarization or a TE (“transverse electric”) polarization.Typically, a WDM signal including first and second optical signalshaving TE and TM polarizations, polarizations, may be supplied to PBS202 via input waveguide 201. Those optical signals (first opticalsignals) having a TE polarization (TE WDM) may be output from a port orwaveguide 205 coupled to polarizer 210 and those having a TMpolarization (TM WDM) may be output from a port of waveguide 207 coupledto polarizer 208. Preferably, each of first optical signals within TEWDM has a corresponding one of a set of wavelengths (λ1 to λn), and eachsecond optical signals within TM WDM has a corresponding of the same setof wavelengths. The set of wavelengths is the same set of wavelengthsassociated with the WDM signal input to polarization demultiplexer 112.

A known polarizer 210 may receive the first TE polarized opticalsignals, for example, and may be configured to block or filter anyextraneous or residual light that does not have the TE polarization. Asa result, a TE WDM signal including the first optical signals is outputfrom polarizer 210. In addition, a known optical rotator 206 may beprovided to rotate the polarization of the second optical signals outputfrom waveguide 207 so that the second optical signals have a TEpolarization, for example. A known polarizer 208 may be further providedto filter or block any extraneous or residual light output from 206 thatdoes not have the TE polarization. The polarization-rotated secondoptical signals, labeled TE′WDM in FIG. 2, are output from polarizer208.

Although FIG. 1 illustrates polarization demultiplexer 112 provided onthe same substrate as, for example, amplifiers 114, demultiplexer anddelay circuitry 116, optical hybrid circuitry 118, and photodiodes 120,it is understood that polarization demultiplexer 112 may be provided ona separate substrate from substrate 110. In that case, polarizationmaintaining fibers may be used to supply optical signals TE WDM and TMWDM to amplifiers 114 and other components on substrate 110.

FIGS. 3 and 4 illustrate examples of optical demultiplexing and delaycircuitry 300 and 400 with may be included in circuitry 116 inaccordance with an example of the present disclosure. Such circuitry mayboth be provided on substrate 110 and each may include a correspondingoptical demultiplexer, such as an arrayed waveguide grating (AWG) 302,402. In FIG. 3, optical signals TE WDM output from polarizationdemultiplexer 112 are supplied to an input waveguide 301 of AWG 302. AWG302 includes free space or slab waveguides (“slabs”) 304 and 308 with aplurality of waveguides 306 connected there between. In addition, aplurality of output waveguides 309-1 to 309-n extend from slab 308. Asgenerally understood, AWG 302 may be configured such that each opticalsignal included in the TE WDM signal is output on a corresponding one ofoutput waveguides 309-1 to 309-n. Each optical signal has, in thisexample, a TE polarization and a corresponding one of the set ofwavelengths λ1 to λn (optical signals TE λ1 to TE λn). Also, eachoptical signal is supplied to a corresponding one of a plurality ofoptical splitters or power splitter 310-1 to 310-n (collective referredto as splitters 310). Each splitter 310 outputs a first portion of areceived optical signal on a corresponding one of waveguides 312-1 to312-n and a second portion to a corresponding one of optical delaycircuits 314-1 to 314-n. In the example, shown in FIG. 3, each of delaycircuits 314-1 to 314-n may include a delay interferometer, whichincludes an additional waveguide segment, e.g., waveguide segment 313-1in delay circuit 314-1, such that optical signals passing through thedelay interferometer traverse a longer optical path than those output oncorresponding waveguide 312-1. Thus, optical signals propagating throughdelay circuits 314-1 to 314-n (TE λ1(Del) to TE λn (Del)) are delayedversions of TE λ1 to TE λn, i.e., TE λ1 (Del) to TE λn (Del) are delayedrelative to TE λ1 to TE λn, respectively. Other known delay circuits mayalso be provided instead of the delay interferometers discussed above.

Preferably, each of optical signals TE λ1 to TE λn carry a series ofbits, such that each bit is transmitted over a period of time referredto as a bit period. In one example, the amount of delay associated witheach delay circuit 414-1 to 414-n is substantially equal to the bitperiod.

In the example shown in FIG. 3, each of splitters 410-1 to 410-nincludes a coupler. However, it is understood that in this example, aswell as the other examples discussed herein, other known power splittersmay be provided instead of the couplers discussed above.

The second optical signals (TE′ WDM discussed above in connection withFIG. 2) are output from polarization demultiplexer 112 and aredemultiplexed, as well as power split into two portions that are delayedrelative to one another by circuitry shown in FIG. 4. The circuitryshown in FIG. 4 includes similar components that operate in a similarfashion as those components discussed above in regard to circuitry 300.Namely, input waveguide 401, AWG 402 (including slabs 404 and 408 andwaveguides 406), output waveguides 409-1 to 409-n, splitters 410-1 to410-n, waveguides 412-1 to 412-n, and optical delay circuits (e.g.,delay interferometers) 414-1 to 414-n operate to output optical signalportions TE′ λ1 to TE′ λn (each having a corresponding one ofwavelengths λ1 to λn) and delayed versions of these optical signalportions as TE′ λ1 (Del) to TE′ λn (Del). Here also, the delayassociated with each of circuits 414-1 to 414-n is substantially equalto a bit period, such that each of TE′ λ1 (Del) to TE′ λn (Del) isdelayed relative to TE′ λ1 to TE′ λn, respectively, by this amount.

As noted above, receiver 100 may be used to receive Pol Muxed DQPSKoptical signals, in which information is encoded by varying the phase ofsuch signals. Accordingly, in order to decode such signals, a phasereference is preferably provided at the receiver. The delayed signals(TE λ1 (Del) to TE λn (Del) and TE′ λ1 (Del) to TE′ λn (Del) discussedabove may provide such a reference, which, along, with the non-delayedsignals TE λ1 to TE λn and TE′ λ1 to TE′ λn are supplied to opticalhybrid circuits 120.

Alternatively, as shown in FIG. 5, a plurality of lasers 512-1 to 512-nmay be provided as local oscillators that generate phase referenceoptical signals TEλ1LO to TE λnLO to realize a coherent receiver ordetector that receives optical signals modulated in accordance with aquadrature phase shift-keying (QPSK) modulation format. As further shownin FIG. 5, signal TE WDM may be supplied to AWG 502, which includesslabs 504 and 508 and waveguides 506. Output waveguides 510-1 to 510-nsupply each of a corresponding one of optical signals TE λ1 to TE λn.These optical signals, along with reference signals TEλ1LO to TE λnLO,are supplied to optical hybrid circuits 118 for further processing. In asimilar fashion, optical signal TE′ WDM is demultiplexed by an AWG (notshown) and the demultiplexed optical signals TE′ λ1 to TE′ λn are alsosupplied to optical hybrid circuits 120. Optionally, a portion of theoutputs of lasers 512-1 to 512-n may be rotated and also supplied tooptical hybrid circuits 118 as reference signals associated with TE′ λ1to TE′ λn. Lasers 512-1 to 512-n may be provided on the same substrateas AWG 502. Additional examples of coherent receivers consistent withthe present disclosure are discussed below in connection with FIGS. 14 band 14 c.

FIG. 6 illustrates an example of demultiplexing and delay circuitry thatmay be used to supply optical signals TE λ1 to TE λn, TE′ λ1 to TE′ λn,TE λ1(Del) to TE λn(Del), and TE′ λ1(Del) to TE′ λn(Del) with one AWG(AWG 602). In the example shown in FIG. 6, input waveguides 601 and 603supply signals TE′ WDM and TE WDM, respectively, from polarizationdemultiplexer 112 to first (input) slab 604 of waveguide 602. AWG 602includes waveguides 606, similar to those discussed above, as well as asecond (output) slab 608.

As generally understood, the location along an input slab at whichoptical signals are input to the AWG determines the locations at whichthe demultiplexed optical signals are output from the AWG. Thus,although TE WDM and TE′ WDM include optical signals having the samewavelengths, since both are input to AWG 602 at different locationsalong slab 604, the demultiplexed optical signals associated with TE WDM(TE λ1 to TE λn) are output at different locations along slab 608 thanthe optical signals associated with TE′ WDM (TE′ λ1 to TE′ λn).Accordingly, AWG 602 may be configured to demultiplex both TE WDM andTE′ WDM.

Thus, as further shown in FIG. 6, the TE WDM signal is input to AWG 602at location 605-1 and the TE′ WDM signal is input to AWG 602 at location605-2. As a result, demultiplexed optical signals TE λ1 to TE λn areoutput on waveguides 609-1 to 609-n, respectively, and TE′ λ1 to TE′ λnare output on waveguides 607-1 to 607-n, respectively. Each ofwaveguides 607-1 to 607-n extends from slab 608 and supplies acorresponding one of optical signals TE′ λ1 to TE′ λn, and each ofwaveguides 609-1 to 609-n supplies a corresponding one of opticalsignals TE λ1 to TE λn. Each of optical signals TE′ λ1 to TE′ λn aresupplied to a corresponding one of splitters 611-1 to 611-n, and each ofoptical signals TE λ1 to TE λn are fed to a corresponding one of opticalsplitters 610-1 to 610-n. Each of splitters 610-1 to 610-n has a firstoutput connected to a corresponding one of optical delay circuits 614-1to 614-n to supply optical signals TE λ1(Del) to TE λn(Del) and a secondoutput connected to a corresponding one of the waveguides 612-1 to 612-nto supply optical signals TE λ1 to TE λn. Likewise, each of splitters611-1 to 611-n has a first output connected to a corresponding one ofoptical delay circuits 615-1 to 615-n to supply optical signals TE′λ1(Del) to TE′ λn(Del) and a second output connected to a correspondingone of the waveguides 613-1 to 613-n to supply optical signals TE′ λ1 toTE′ λn.

FIG. 7 illustrates another example in which optical signal TE WDM is fedto one slab (704) of AWG 702 (including waveguides 706) and thecorresponding demultiplexed optical signals (TE λ1 to TE λn) are outputfrom the opposite slab (708). In a similar fashion, TE′ WDM is fed toslab 708 and the corresponding demultiplexed optical signals TE′ λ1 toTE′ λn are output from the opposite slab 704. Thus, in FIG. 7, TE WDMand TE′ WDM are fed to opposite slabs, whereas, in FIG. 6, these signalswere supplied to the same slab.

Namely, each of demultiplexed optical signals TE λ1 to TE λn (includedin signal TE WDM) is output on a corresponding one of waveguides 709-1to 709-n extending from slab 708 (opposite slab 704), and each ofdemultiplexed optical signals TE′ λ1 to TE′ λn (included in signal TE′WDM) is output on a corresponding one of waveguides 707-1 to 707-nextending from slab 704 (opposite slab 704). Splitters 710-1 to 710-nand 711-1 to 711-n; waveguides 712-1 to 712-n and 713-1 to 713-n; andoptical delay circuits 714-1 to 714-n are configured in a manner similarto that described above in connection with the splitters, waveguides anddelay circuits shown in FIG. 6. As in the example illustrated in FIG. 6,the circuitry shown in FIG. 7 outputs optical signals TE λ1 to TE λn;TE′ λ1 to TE′ λn; TE λ1(Del) to TE λn(Del); and TE′ λ1(Del) to TE′λn(Del).

FIGS. 8-11 illustrate additional examples of optical demultiplexer anddelay circuitry wherein the WDM signals are power split and delayedprior to demultiplexing. An advantage of the circuits shown in FIGS.8-11 is that fewer delay circuits are required compared to the circuitrydiscussed above in connection with FIGS. 3-7. Since delay circuits arerelatively large, the circuitry shown in FIGS. 8-11 may be reduced insize relative to the circuitry shown in FIGS. 3-7.

In the example shown in FIG. 8, waveguides 801 and 803 receive opticalsignals TE WDM and TE′ WDM, respectively. Splitter 81, which may have astructure similar to the splitters discussed above, is coupled towaveguide 801 and supplies a first portion to slab 804 of AWG 802, whichincludes waveguides 806. A second portion of the TE WDM signal issupplied to delay circuit 814, which may have a delay interferometersimilar to that discussed above. As further noted above, the delayinterferometer may delay the TE WDM signal by a bit period, for example.The non-delayed output of splitter 813 and the output of delay circuit814 are both supplied to slab 804 of AWG 802.

Splitter 815 may have a structure similar to that described above tosplit the incoming TE′ WDM signal into two portions: a first portionthat is supplied directly to slab 804; and a second portion that isdelayed by delay circuit 816 (including a delay interferometer, forexample), typically by a bit period.

The delayed and non-delayed portions of TE WDM and TE′ WDM aredemultiplexed by AWG 802, such that each of optical signals TE λ1 to TEλn are output on a corresponding one of output waveguides 820-1 to820-n; each of optical signals TE′ λ1 to TE′ λn are output on acorresponding one of output waveguides 824-1 to 824-n; each of opticalsignals TE λ1(Del) to TE λn(Del) are output on a corresponding oneoutput waveguides 822-1 to 822-n; and each of optical signals TE′λ1(Del) to TE′ λn(Del) are output on a corresponding one of outputwaveguides 826-1 to 826-n.

In the configuration shown in FIG. 8, AWG 802 may have a relativelylarge number of outputs waveguides, which may require that the size ofAWG 802 may be relatively large.

FIGS. 9 and 10 illustrate another example of demultiplexing and delaycircuitry 116 in which delayed and non-delayed portions of TE WDM aredemultiplexed by a first AWG (902) and delayed and non-delayed portionsof TE′ WDM are demultiplexed by a second AWG (1002). Both AWGs 902 and1002 may be provided on the same substrate, e.g., substrate 110, whichmay include InP, for example.

In FIG. 9, TE WDM is supplied on an input waveguide 901 to splitter 913,which is similar to the splitters discussed above. Splitter 913 suppliesa first portion of TE WDM to slab 904 and a second portion to delaycircuit 914 (similar to the delay circuits discussed above), whichdelays the second portion, typically, by a bit period. The delayedsecond portion is also supplied to slab 904. AWG 902, which includesadditional waveguides 906, demultiplexes the optical signals thatconstitute the first portion an the delayed second portion of TE WDM,such that each of TE λ1 to TE λn is output on a corresponding one ofwaveguides 920-1 to 920-n, and each of TE λ1(Del) to TE λn(Del) isoutput on a corresponding one of waveguides 922-1 to 922-n.

The delay and demultiplexing circuit shown in FIG. 10 also includes aninput waveguide 1001, splitter 1013, delay circuit 1014, AWG 1002(including waveguides 1006 and slabs 1004 and 1008), and outputwaveguides 1020-1 to 1020-n and 1022-1 to 1022-n. The circuitry shown inFIG. 10 operates in a similar fashion as that shown in FIG. 9 to supplyoptical signals TE′ λ1 to TE′ λn and TE′ λ1(Del) to TE′ λn(Del).

In the examples shown in FIGS. 9 and 10, the first and delayed secondportions of the incoming WDM signals are supplied to the same AWG slab.In the examples shown in FIGS. 11 and 12, however, the first and secondWDM portions are supplied to opposite slabs, and, as a result, thedemultiplexed optical signals are output from different slabs.

For example, as shown in FIG. 11, TE WDM is supplied on waveguide 1101to splitter 1113 (similar to the splitters discussed above), whichsupplies a first portion to of TE WDM to slab 1104 and a second portionto delay circuit 1114 (having a structure similar to of the delaycircuits described above). Delay circuit 1114 typically delays thesecond portion of TE WDM by a bit period relative to the non-delayedfirst portion and supplies the delayed second portion to slab 1108opposite slab 1104. As a result, each of the optical signals thatconstitute non-delayed first portion of TE WDM (TE λ1 to TE λn) isdemultiplexed by AWG 1102 and output on a corresponding one ofwaveguides 1120-1 to 1120-n, and each of the optical signals thatconstitute the delayed second portion of TE WDM (TE λ1(Del) to TEλn(Del)) is output on a corresponding one of waveguides 1122-1 to1122-n.

The circuitry shown in FIG. 12 (waveguide 1201, splitter 1213, delaycircuit 1214, AWG 1202 including waveguides 1206 and slab 1204 and1208), waveguides 120-1 to 1220-n, and waveguides 1222-1 to 1222-n) issimilar to that shown in FIG. 11 and operates to supply optical signalsTE λ1 to TE λn, TE′ λ1 to TE′ λn, and their delayed versions in a mannersimilar to that discussed above in regard to FIG. 11.

FIG. 13 is a detailed diagram of an exemplary receiver circuit 1300incorporating the optical demultiplexing and delay circuitry shown inFIGS. 11 and 12. Receiver circuit 1300 may be provided or integrated ona substrate 1310, which may include indium phosphide (InP), for example.Receiver circuit 1300 includes a polarization beam splitter (PBS) thatreceives an incoming WDM signal including a plurality of opticalsignals, each at a corresponding one of a plurality of wavelengths. Asdiscussed above in connection with FIG. 2, the PBS may have two outputs,which supply a first portion of the WDM signal (TE WDM) to a polarizerPol1 and a second portion of the WDM signal (TM WDM) to a rotator andpolarizer Pol2. Typically, as noted above, the first optical signalsthat constitute the first portion of the WDM signal have a firstpolarization, e.g., a TE polarization, and the second optical signalsthat constitute the second portion of the WDM signal have a secondpolarization, e.g., a TM polarization.

As further noted above in connection with FIG. 2, after passing throughthe rotator and polarizer (P2), the polarization of each of the secondoptical signals in TM WDM is rotated so that each has a TE polarization.Accordingly, the second optical signals are designated TE′ WDM in FIG.13, as in the above discussion in connection with FIG. 2.

The first WDM signal portion TE WDM is supplied to an optical coupler,splitter or tap TAP1, which power splits TE WDM a first part, which mayhave less power (e.g., 10% of the light input to tap TAP1) than a secondpart that is supplied directly to input IN12 of arrayed waveguidegrating AWG1. The first part of TE WDM is fed to a delay waveguide BD1that introduces a delay which is typically equal to one bit period. Thedelayed signals (WDM TE (Del)) may then be amplified by an optionalsemiconductor optical amplifier SOA1 and then fed to input waveguideIN11, which extends from free space region or slab waveguide FS11 ofAWG1. AWG1 is configured to demultiplex WDM TE(Del), such that eachwavelength component (e.g., TE λ1(Del)) thereof is output on acorresponding of waveguides OUT12. AWG1 is further configured such thateach wavelength component (e.g., TE λ1) of the second part of the firstWDM signal portion (WDM TE) supplied to input IN12 is output on each ofwaveguides OUT12. In the example shown in FIG. 13, each optical signaloutput from AWG1 typically has a TE polarization, as noted aboveconnection with FIG. 11.

In a similar fashion, the second WDM signal portion WDM TE′ (output formpolarizer Pol2) is split by coupler or tap Tap2. A first part of WDM TE′(Del) is then delayed by delay waveguide BD2, amplified by semiconductoroptical amplifier SOA2, and supplied to input IN21 extending to freespace region FS21 of AWG2. A second part of the second WDM signalportion (WDM TE′) is fed to input IN22 of AWG2. In the example shown inFIG. 13, WDM TE′(Del) is demultiplexed into optical signals (wavelengthcomponents, such as TE′ λ1(Del)) that are output on waveguides OUT21 andthe second part of the second WDM signal, TE′ WDM, is demultiplexed intofurther optical signals (e.g., TE′ λ1) that are output on waveguidesOUT22.

As further shown in FIG. 13, the demultiplexed optical signals from AWG1and AWG2 are next fed to 90 degree optical hybrid circuits (OHCs), whichare known circuits that output sums of the input optical signalsthemselves, as well as the sums of the input signals and phase shifted,e.g., by 90 degrees, versions of the input signals. As generallyunderstood, the polarization states of the optical signals supplied toeach OHC are preferably be the same in order to insure that the datacarried by the optical signals is adequately detected. Accordingly, asnoted above, optical signals in WDM TM are rotated, for example, so thateach of the optical signals supplied to the optical hybrid circuits OHCstypically have the same polarization, e.g., a TE polarization.

Photodiodes PDs convert the received optical output from optical hybridcircuits OHCs to corresponding electrical signals. Photodiodes PDs maybe arranged in pairs, such as PD1 and PD2, and connected to one anotherin a balanced configuration. The output of each balanced pair (PD1 andPD2) supplies one of a quadrature (Q) or in-phase (I) electrical signal,which is amplified by one of transimpedance amplifiers (TIAs) and theamplified Q and I signals are combined and fed tomultiple-input-multiple-output (MIMO) circuits M11, M12, M21, and M22,examples of which are described in the above-noted U.S. patentapplication Ser. No. 12/052,541. It is understood that the OHCs, PDs,and TIAs shown in FIG. 13 are provided to receive and process opticalsignals at a given wavelength. Optical signals at other wavelengths thatare output from AWG1 and AWG2 are received by OHCs, PDs, and TIAs havinga structure similar to that shown in FIG. 13.

Receiver 1300 has a relatively compact design, since AWGs 1 and 2 areconfigured to both receive and output optical signals through slabwaveguides or free space regions (FS11, FS12, FS21, and FS22). Moreover,each of the above-noted components may be provided on substrate 1310.

FIG. 14 a shows an embodiment similar to that shown in FIG. 13, exceptthat each of a plurality of delay circuits (e.g., BD 14 and BD15) isprovided for a corresponding one of outputs of AWG 1405. Although oneAWG may be provided in the example shown in FIG. 14, a delay circuit isprovided for each AWG output (e.g., 1420-1 to 1420-n and 1422-1 to1422-n), as discussed above in connection with FIG. 7.

FIG. 14 b illustrates an example similar to that shown in FIG. 14 a. Inthe example shown in FIG. 14 b, however, alternative polarizationdemultiplexing circuitry 112 is shown as including a polarization beamsplitter (PBS) and rotator. In addition, first, second, and thirdpolarizers P1, P2, and P3 are provided instead of two, as noted above inconnection with FIGS. 2 and 13. As further shown in FIG. 14 b, opticalsignals TM WDM and TE WDM are output from first and second outputs,respectively, from the PBS. A first polarizer acts as a filter to passlight having a TM polarization, but blocks light having otherpolarizations, such as a TE polarization. The rotator then rotates thelight output from polarizer P1, so that such light has a TEpolarization. A second polarizer P2 is provided to filter light having apolarization other than the TE polarization, and thus the opticalsignals in TM WDM are each output to the demultiplexing circuitry, whichmay be an AWG, as shown in FIG. 14 b, with a TE polarization and are sodesignated TE′ WDM. A third polarizer P3 is configured to pass lighthaving a TE polarization, and may be included to block light at otherpolarizations that may be output with TE WDM from the polarization beamsplitter. In FIG. 14 b, each of the polarized optical signals in TE WDMand TM WDM (and TE′ WDM) may be modulated in accordance with adifferential quadrature phase shift-keying (DQPSK) format.

Further in FIG. 14 b, the waveguides of the optical hybrid circuits OHCsshown in FIG. 14 a are replaced by multi-mode interference (MMI)couplers MMI1 to MMI4. The outputs of MMI couplers MMI1 to MMI4 are nextfed to photodiodes PD in a manner similar to that discussed above.

FIG. 14 c illustrates another example including the polarizationdemultiplexer shown in FIG. 14 b. In FIG. 14 c, however, delay circuits,such as BD14 and BD15, are omitted. Instead a plurality of localoscillators, similar to those discussed above in connection with FIG. 5,are provided so that a coherent receiver can be realized. One such localoscillator is shown including laser LOλ1, which outputs light atwavelength λ1. LOλ1 may include a photonic bandgap laser, such as adistributed feedback (DFB) laser. Typically, local oscillator laser LOλ1may operate at a relatively high power so that the light outputtherefrom is within a relatively narrow spectral range or has arelatively narrow line width. Accordingly, light output from LOλ1 may bepassed through an optional optical isolator ISO and a variable opticalattenuator or fixed loss (FL) element 1492 (such as a power splitter ortap) and then fed to splitter or coupler C. A first output of coupler Csupplies a first portion of the light from local oscillator LOλ1 to afirst MMI coupler 1498 and a second output of coupler C supplies asecond portion of the light from local oscillator LOλ1 to a second MMIcoupler 1499. Optional VOA/FLs 1494 and 1496 may be coupled to the firstand second outputs of coupler C, respectively, instead of or in additionto VOA/FL 1492. VOA/FL 1492 and/or VOA/FLs 1494 and 1496 may be providedto regulate the optical power of local oscillator LOλ1 to appropriatelevels, while maintaining a relatively narrow line width. In FIG. 14 c,each of the polarized optical signals in TE WDM and TM WDM (and TE′ WDM)may be modulated in accordance with a quadrature phase shift-keying(QPSK) format.

Since the local oscillators, e.g., local oscillator LOλ1, typicallyoutput light having a TE polarization, the optical signals in TM WDM,are rotated to have the TE polarization, and the optical signals in TEWDM have the TE polarization without rotation. As a result, each ofoptical signals (e.g., those in TE WDM, TE′ WDM, and the localoscillator outputs) supplied to the optical hybrid circuits OHCs is thesame, which as noted above in connection with FIG. 13, may be requiredfor adequate data detection.

Optical hybrid circuitry including MMI couplers 1498 and 1499 receivethe demultiplexed outputs from the AWG shown in FIG. 14 c as well as thepower adjusted outputs of the local oscillator LOλ1, and operate inmanner similar to that described above to provide optical signals tophotodiodes PD, which, in turn, supply corresponding electrical signalsto transimpedance amplifiers TIA. For convenience, the MIMO circuitry isnot shown in FIG. 14 c.

As further shown in FIG. 14 c, heater H1, such as a know thin filmheater, may be provided to adjust a temperature, and thus a wavelengthof light output from LOλ1. In addition, if VOAs are included in elements1492, 1494, and 1496, heaters H2, H3, and H4 may be provided in or toadjust the attenuation of such VOAs (i.e., vary the amounts of portionsof the input light that are output from the VOAs) in a manner similar tothat described below with reference to FIG. 14 d. The VOAs may include aforward biased semiconductor optical amplifier or a photodiode, such asa PIN photodiode. In addition, a photodiode BPIN may be provided tosense light output from a back facet of LOλ1 in order to determine theoptical power output therefrom. In that case, light output from thefront facet of LOλ1 is supplied to isolator ISO, as noted above. Thecircuitry discussed above (e.g., the AWG, BPIN, local oscillators, VOAs,optical hybrid circuitry, and photodiodes) may be provided on asubstrate, such as substrate 110 shown in FIG. 1.

FIG. 14 d illustrates an example of a VOA 1605, which may be used as oneor more of the VOAs discussed above in connection with FIG. 14 c. VOA1605 is configured as a Mach-Zehnder interferometer in this example, andincludes an input waveguide 1610 configured to receive an opticalsignal, such as the output from local oscillator LOλ1. Input waveguide1610 supplies such light to a first coupler 1613, which, in turn, feedsa first portion of the optical signal to a first branch waveguide 1612and a second portion of the optical signal to a second branch waveguide1614. A second coupler is also provided that is configured to combinefirst parts of the first and second portions of the optical signal ontoa first output waveguide 1640, and second parts of the first and secondportions of the optical signal onto a second output waveguide 1620. Thefirst parts constituting a first combined optical signal and the secondparts constituting a second combined optical signal. Typically, anoptical power associated with the first combined optical signal isgreater, and may be substantially greater, than the optical powerassociated with the second combined signal.

A heater 1642, such as a thin film heater including porous silicon,platinum or other metal or alloy, for example, may also provided thatadjust the temperature of the first branch waveguide 1612, for example,in response to an applied bias. As is generally known, changes intemperature in the first branch waveguide 1612 cause changes in therefractive index of this waveguide, which results in changes in phase ofthe light propagating there through. Accordingly, as generallyunderstood, such phase changes result in variations in the optical powerof light supplied to output waveguide 1640.

Although VOA 1605 is configured such that a substantial part of theinput signal is output on waveguide 1640, a relatively small part of theinput signal may be output on waveguide 1620 due to process variationsand/or other non-idealities. Such residual or extraneous light mayinterfere with light in waveguide 1640, especially if VOA 1605 isintegrated on substrate. Accordingly, consistent with an aspect of thepresent disclosure, photodiode 1630 may be provided to receive andabsorb the residual light, thereby minimizing interference with light inwaveguide 1640. Photodiode 1630 is preferably biased so that anode1630-1 is coupled to ground and cathode 1630-2 is coupled to a voltageor potential V. Preferably VOA 1605 is provided on a substrate, such assubstrate 110.

FIG. 14 e shows another example of a coherent receiver, in which couplerC and the BPIN photodiode are omitted. Here, LOλ1 includes first andsecond facets F1 and F2, which light first and second light,respectively. The first light is supplied to VOA/FL 1494 and the secondlight is supplied, via optical isolator ISO, to VOA/FL 1496. Theattenuation of the first and second light may be adjusted or controlledby heaters, e.g., thin film heaters, H2 and H3, respectively. VOA/FL1494 and VOA/FL 1496 supply power adjusted optical outputs to opticalhybrid circuitry including MMI couplers 1498 and 1499 in a mannersimilar to that discussed above. The operation and structure of thewaveguides WG, photodiodes PD, and transimpedance amplifiers TIAs, aswell as the polarizers P1, P2, P3, demultiplexer circuitry (AWG), andthe polarization beam splitter PBS, is similar to that discussed above.In addition, the circuitry shown in FIG. 14 e may be integrated on acommon substrate, such as substrate 110. Moreover, it is understoodthat, as in FIG. 14 c, a plurality of local oscillators may be provided,each of which being associated with a corresponding one of the opticalsignal wavelengths present in TE WDM and T E′WDM. Thus, as in FIG. 14 c,the local oscillator, isolator ISO, VOA/FL and MMI, photodiode (PD) andTIA components shown in FIG. 14 e may be similarly provided for eachoptical signal present in TE WDM and TE′WDM that is demultiplexed by theAWG. In addition, each of the polarized optical signals in TE WDM and TMWDM (and TE′ WDM) may be modulated in accordance with a QPSK format.

As further shown in FIG. 14 e, optional semiconductor optical amplifiers(SOAs) 1437 and 1439 may be provided at the inputs to polarizers P2 andP3. SOAs 1437 and 1439 primarily amplifier TE polarized light. As such,each may be used to suppress or filter any residual TM light that may bepresent in the output of the rotator and the polarization beam splitter(PBS). In addition, although SOAs 1437 and 1439 may output a relativelysmall amount of amplified stimulated emission (ASE) light having a TMpolarization, such TM ASE light may be filtered by polarizers P2 and P3.It is understood that SOAs 1437 and 1439 may be similarly coupled topolarizers P2 and P3 in FIGS. 14 b and 14 c.

FIG. 15 shows an example of an AWG 1510, which may be used as amultiplexer at a transmit end of a WDM system. AWG 1510 receives inputsignals having a first polarization from sources 1-3 at inputs (e.g.,input waveguides) IN31 of free space region FS1. The input signals arecombined into a WDM signal that is supplied to output waveguide OUT31coupled to free space region FS2. Similarly, further input signalssupplied from sources 1-3 are fed to inputs (e.g., input waveguides)IN32 coupled to free space region FS2, and these signals are combined asanother WDM signal supplied from output waveguide OUT32. Preferably,each of the input signals supplied to input waveguides IN31 has acorresponding one of a plurality of wavelengths, but the samepolarization, e.g., a TE polarization. In addition, each of the inputsignals supplied to input waveguides IN32 has a corresponding one of theplurality of wavelengths, but a polarization different than thepolarization of optical signals supplied to inputs IN31. For example,each of the signals supplied to input waveguides IN32 may have a TMpolarization. Sources 1-3 may have a structure similar to that discussedin U.S. patent application Ser. Nos. 12/345,315 and 12/363,826, theentire contents of both of which are incorporated herein by reference.In particular, each of sources 1-3 may include a laser having first andsecond facets. The first facet may output a first optical signal that ismodulated to carry first information, and the second facet may output asecond optical signal that is modulated to carry second information. Thefirst and second optical signals may have the same wavelength.

Receivers consistent with the present disclosure may have a higher RFbandwidth and do not suffer from degradations due to multiple levels ofphotodiode assembly and wire bonding. In addition, mechanical assemblyof a first substrate to a second substrate (e.g. a photonic ligthwavecircuit (PLC) to an InP substrate) is unnecessary. Moreover, receiversconsistent with the present disclosure may have relatively low RF (radiofrequency) noise in the photodiodes (PDs) because fewer wire bonds andleads are required. Further, if provided on an InP substrate, theabove-described receivers may be more easily tunable than componentsprovided on a silica based substrate, such as those commonly used toprovide a photonic lightwave circuit. In addition, the above-describedconfigurations may be used in both coherent and direct detectionreceivers, as well with optical signals having various modulationformats, such as DQPSK and QPSK.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. An apparatus, comprising: a polarization beam splitter having aninput waveguide and first and second output waveguides, the polarizationbeam splitter being configured to receive, at the input waveguide, afirst optical signal having a first polarization and a second opticalsignal having a second polarization different than the firstpolarization; a first polarizer configured to pass light having thefirst polarization, the first polarizer being configured to receive thefirst optical signal from the first output of the polarization beamsplitter; a rotator configured to receive the first optical signal fromthe first polarizer, the rotator being configured to output the firstoptical signal so that the first optical signal has the secondpolarization; a second polarizer configured to pass light having thesecond polarization, the second polarizer being configured to receivethe first optical signals having the second polarization; a thirdpolarizer configured pass the light having the second polarization, thethird polarizer configured to receive the second optical signal havingthe second polarization, the first optical signal is one of a pluralityof first optical signals and the second optical signal is one of aplurality of second optical signals, each of the plurality of firstoptical signals having a corresponding one of a plurality ofwavelengths, and each of the plurality of second optical signals havinga corresponding one of the plurality of wavelengths, each of theplurality of first optical signals is output, with the secondpolarization, from the second polarizer and each of the plurality ofsecond optical signals is output, with the second polarization, from thethird polarizer; and optical demultiplexer circuitry, the opticaldemultiplexer circuitry having first and second inputs, the first inputof the optical demultiplexer circuitry being configured to receive theplurality of first optical signals from the second polarizer and thesecond input of the optical demultiplexer circuitry is configured toreceive the plurality of second optical signals from the thirdpolarizer.
 2. An apparatus in accordance with claim 1, further includingoptical hybrid circuitry configured to receive the first and secondoptical signals from the second and third polarizers, respectively. 3.An apparatus in accordance with claim 1, further including a pluralityof photodiodes coupled to the optical hybrid circuitry.
 4. An apparatusin accordance with claim 3, wherein the optical hybrid circuitryincludes a multi-mode interference (MMI) coupler.
 5. An apparatus inaccordance with claim 1, further including optical hybrid circuitryconfigured to receive the first and second optical signals.
 6. Anapparatus in accordance with claim 5, wherein the optical hybridcircuitry includes a multi-mode interference (MMI) coupler.
 7. Anapparatus in accordance with claim 1, further including a first opticalamplifier coupled to the rotator and second polarizer and a secondoptical amplifier coupled to the first polarizer.
 8. An apparatus inaccordance with claim 7, wherein the first optical amplifier includes afirst semiconductor optical amplifier and the second optical amplifierincludes a second semiconductor optical amplifier.
 9. An apparatus inaccordance with claim 1, further including a substrate, each of thepolarization beam splitter, the rotator, and the first, second, andthird polarizers being provided on the substrate.